In UMTS (Universal Mobile Telecommunication System), the channelisation code dictates the bandwidth allocated to a particular service. In the uplink, each mobile station has its own channelisation code (or scrambling code) tree, since different uplink scrambling codes are allocated to different mobile stations. The code tree allocated to a particular mobile station will be shared among different radio access bearers (i.e. base stations) of that mobile station. However, with service multiplexing multiplexing of transport channels), either a single channelisation code (for spreading factor less than 4) is used or multiple channelisation codes (also known as multicode, for high data rate) are used. The uplink channelisation code is allocated in a predefined order. The mobile and the network (i.e. the base station) only need to agree on the code number (which may be predefined) and spreading of the codes. The spreading factor is implicitly given in the transport format combination indicator (TFCI) on the dedicated physical control channel (DPCCH). The exact codes to be used are thus implicitly given. Fast symbol rate change (at a resolution of 10 ms radio frame duration) is therefore possible on the uplink as long as the TFCI is decoded properly. No radio access bearer (RAB) modification is thus needed. However, due to the use of predefined channelisation code, the cross-correlation among, mobile stations may degrade and thus increase the interference level.
In the downlink, the channelisation code tree is shared among different mobile stations within a cell. This results in a downlink channelisation code shortage problem. A downlink channelisation code can only be allocated if and only if no other code on the path from the specific code to the root of the tree or in the sub-tree below the specific code is used within the same cell This restriction on channelisation code allocation applies as well to the uplink except that it is for a mobile station and not for a cell. Moreover, the allocation of uplink channelisation code can be easily predefined in such a way as to ensure that the restriction is not violated. As for the downlink channelisation allocation can be quite complicated since it has to share the channelisation code tree among different mobile stations. A change in spreading factor in the downlink requires performing the RAB modification procedure. This is because the DPCCH and the dedicated physical data channel (DPDCH) are time-multiplexed and thus use the same channelisation code. Decoding the TFCI in the DPCCH requires knowledge of the spreading factor in advance. Hence, a 10 ms symbol rate resolution change is not possible. A symbol rate change of 10 ms is still possible through dynamic rate matching, however the spreading factor is constant (i.e. set to lower spreading factor than is required for the physical channel). The problem is summarised as follows. In the uplink, there is an increase in in interference level due to the use of predefined channelisation code among mobile stations.
In the downlink, there is a complicated code allocation algorithm to optimise the use of the channelisation code tree, a slow spreading factor changes due to time-multiplexing of the DPCCH and DPDCH, and a code shortage problem due to a single code tree among different mobile stations within a cell.
It is therefore an object of the present invention to provide a technique for allocating a code branch in the uplink or in the downlink of a spread spectrum CDMA system which results in an improved system performance.
It is known from International Patent Publication WO95/03652 to provide a method of allocating a channelisation code in a code division multiple access system comprising for each user: selecting a node of a path in a code tree in dependence on the currently required spreading factor; and reserving all nodes on the code tree in an upward and downward direction from the selected node; wherein the selected node for any user must not coincide with a node reserved by any other user.